A hollow cathode is an electron beam source which discharges a relatively large amount of thermal electrons through plasma discharge and has a long lifespan.
The conventional industrial hollow cathode, externally, has a tube shape made of a refractory metal, and has an insert (in general, tantalum foil) discharging thermal electrons into the inside of the tube of a single material (in general, tantalum) and multi-layered thermal radiation shielding layers to maintain the insert at a high temperature.
Also, the hollow cathode for an electric condenser for artificial satellites comprises a tube of a refractory metal material, an insert having a low work function (for example, LaB6), a heater for heating the insert and reducing heat loss, and a thermal radiation shielding layer.
The hollow cathode comprises a keeper (electrode) of a disc shape with small holes (for example, a diameter of 0.8 mm) in front of the cathode to discharge thermal electrons, and a body aligning the center axis of the cathode and the keeper and maintaining them at a certain distance.
In order to operate the hollow cathode, power (starter power supply) for outputting a high voltage for initial discharge of plasma between the cathode and keeper, power (keeper power supply) for maintaining the discharge, and a gas providing unit provided to the cathode for plasma discharge are necessary.
Here, in order to apply a negative potential to the cathode for plasma discharge, an electric insulator is used to connect it to the gas providing unit and fix the cathode. In particular, in general an insulation tube (cryogenic break) is used for insulation connection of the cathode with the gas providing line.
Meanwhile, industrial equipment require maintenance at predetermined intervals, and accordingly, the industrial cathode line is inevitably exposed to the atmosphere (in particular, water and oxygen) repetitively. The performance of substances having low work function (for example, LaB6) is greatly deteriorated because the work function increases due to change in physical properties by exposure to the atmosphere.
Thus, the hollow cathode using an insert made of a material with low work function is mainly applied to artificial satellites where there is no concern to be exposed to the atmosphere, and it can operate stably for a long period of time (at least 10,000 hours). However, when applied for industrial use, continuously exposed to the atmosphere, its lifespan drops sharply (to 1,000 hours or below).
The industrial hollow cathode uses a refractory metal (for example, titanium, tungsten, rhenium, hafnium, etc.) stable to exposure to the atmosphere instead of substances having low work function (US 2004/0000853 A1, U.S. Pat. No. 4,339,691 Jul. 13, 1982). In general, tantalum is used in consideration of work function, melting point, thermal conductivity and cost of material, etc.
For the hollow cathode using LaB6 (work function: 3 eV or below) which has a representatively low work function as an insert, there is a possibility that the insert may be thermally damaged by the sharp temperature rise. Thus, the temperature is raised gradually using a heater.
In comparison, the industrial hollow cathode uses refractory metal which is thermally stable. It is possible to raise temperature sharply through plasma discharge by designing an insert with low specific heat (for example, thin foil with a thickness of 0.013 mm), allowing initial discharge using relatively less power.
However, since it has a relatively high work function (as for tantalum, 4.1 eV), it is requested to reach a high temperature (as for tantalum, at least 2,400 K) to discharge thermal electron. This is because the amount of thermal electrons discharged per unit area according to the Richardson's law is T2 exp(−W/kBT), and it has an index relation with the work function value. As the required temperature gets higher, the heat loss by thermal radiation greatly increases in proportion to 4 square of the temperature (Stephan Boltzmann Law: Thot4−Tcold4), and the required power increases sharply.
Thus, the hollow cathode reduces heat loss and power required and enhances thermal stability using a thermal radiation shielding layer of a refractory metal material (in general, tantalum).
However, although the conventional art partly reduces heat loss using a thermal radiation shielding layer, it has a structure that does not sufficiently reduce the heat loss by thermal conduction. This is because a tube of a single material is used, and this has a limitation in reducing heat loss by thermal conduction in terms of production possibility and thermal stability.
For example, a hollow cathode having the same insert does not operate in the same plasma discharge condition when using a molybdenum (thermal conductivity: 138 W/mK) tube which is a metal having a higher thermal conductivity than tantalum (thermal conductivity: 57.5 W/mK), and this is the main reason of the increase in heat loss by conduction.
Using titanium, which is a substance having a relatively lower thermal conductivity (21.9 W/mK) as a tube is advantageous in terms of heat loss, and in terms of cost of material, titanium is advantageous in that it is at least 20 times cheaper than tantalum. However, it cannot be used due to its low melting point (1,941 K).
In comparison, hafnium has a low thermal conductivity (23 W/mK), a low work function (3.9 eV) among the refractory metals, and a sufficiently high melting point (2,506 K). Thus, although it is effective in being applied as a tube and insert of the cathode, due to the extremely expensive cost of the material, it has low competitiveness in the product market when applied to industrial cathodes.
Thus, although the conventional industrial hollow cathode uses tantalum the most based on its physical properties and cost, there is an unnecessary presumption that the cathode should be made of a single material which limits the optimization of the cathode.
Also, the hollow cathode is operated by the principle of raising the temperature of the insert and discharging thermal electrons using plasma discharge (ion bombardment). In this regard, the conventional industrial hollow cathode generally uses a thin refractory metal foil (insert).
In order for the insert to maintain a high temperature, constant heat is delivered from the plasma. This is because of the physical damage caused by continuous ion bombardment with the insert. In particular, thin foil is easily damaged by ion bombardment, deteriorating thermal radiation shielding efficiency. Thus, more power is required and effective area discharging thermal electrons is reduced, thereby reducing the amount of current discharging thermal electrons under the same discharge condition.
In this case, in order for initial discharge of the hollow cathode, a high potential difference is applied between the cathode and keeper (U.S. Pat. No. 5,075,594 Dec. 24, 1991). In this regard, the conventional industrial cathode makes an insulation connection (cryogenic break) with the gas providing line outside the body, so that a large potential is exposed to the outside. Thus, there are problems in electric stability, etc. such that plasma is discharged in the outside.
Also, since the power applying high potential difference (starter power supply) is only used for initial discharge, it is not economic in terms of its usage.
Referring to this in more detail with reference to the attached drawings, FIG. 1 is a cross sectional view illustrating the industrial hollow cathode according to an embodiment of prior art. The industrial hollow cathode 1 comprises a tube 1a and an insert 1b made of tantalum.
Here, the insert 1b may comprise at least one layer (for example, 15 layers) of a thin (for example, a thickness of 0.013 mm) tantalum foil, and perform the role of a thermal radiation shielding layer with respect to the surface discharging inner thermal electrons.
Also, the gas injected from the gas injection hole 1c passes through the insert 1b and gets out of the tube 1a through a small hole 1d, and passes through the cathode through a small hole of the keeper 1e in front of the tube 1a. 
In this case, the center axis if of the tube 1a and the keeper 1e hole is arranged to be on the same line so that the thermal electron generated from the tube 1a is easily discharged outside the hollow cathode 1.
FIG. 2 illustrated next is a result calculating the temperature of the industrial hollow cathode according to an embodiment of prior art. It is the result of calculating the temperature in thermal equilibrium state under a condition providing a constant heat source in a plasma discharged between the industrial hollow cathode 1 and keeper 1e by using the equivalent circuit model (ECM) with respect to the heat flow based on the finite element method.
Here, the plasma heat source (35 W) is determined based on a condition where the maximum temperature reaches 2,400 K when the thermal conductivity of the cathode tube 1a is 50 W/mK which is similar to tantalum.
FIG. 3 illustrated next is a result calculating the temperature distribution of the hollow cathode using a tube 1a made of molybdenum in the industrial hollow cathode structure 1 according to an embodiment of prior art.
Here, the thermal conductivity of molybdenum is about 140 W/mK, and due to the relatively high thermal conductivity, the maximum temperature calculated under the same plasma condition (heat source of 35 W) is 2,200 K. In order to reach a temperature of at least 2,400 K, a higher heat source is required.
In case of using a tube made of molybdenum as an experiment, it does not operate under the same discharge condition, and this shows that selecting the material of the cathode tube is important.
FIG. 4 illustrated next is a result calculating the electron discharge probability distribution in the industrial hollow cathode according to an embodiment of prior art.
The density of the thermal electron discharge current is calculated based on the Richardson's Law(J=AT2 exp(−U/kBT), and here A is an invariable, T is the surface temperature of the cathode, kB is the Boltzmann constant, and U is the work function. That is, the density is calculated based on the surface temperature of the cathode calculated using ECM and the work function (4.1 eV) of tantalum.
As can be seen in the calculated distribution, it may be confirmed that most thermal electrons are discharged from a couple of the thin foils of the innermost part. Also, since the total thermal electron current discharged increases in proportion to the effective surface area, it may be confirmed that the performance may be enhanced by increasing the effective surface area of the inserted located in the inside.
FIG. 5 illustrated next is a cross sectional view illustrating a cross section of the insert of the industrial hollow cathode according to an embodiment of prior art. The inner insert 1b-1 constituting the innermost surface of the insert 1b is directly exposed to ion bombardment generated by plasma discharge, and thus may be damaged. Due to the damage, the effective surface area discharging thermal electrons is reduced and performance deteriorates.
That is, the conventional technology adopts the form of a thin foil (for example, a thickness of 0.013 mm), and thus may be easily damaged by ion bombardment. Thus, there is a request to enhance its performance.
Also, as for the conventional insert 1b, since the thin foil layer 1b-2 of the outer part is exposed to ion bombardment due to the damaged inner insert 1b-1 and thus the performance of the thermal radiation shielding layer deteriorates, the calorie required to maintain temperature increases, and thus the performance of the cathode deteriorates.
(Patent document 0001) Korean Patent No. 10-0899549
(Patent document 0002) Korean Patent No. 10-0925015